the deficit in half. unfortunately, he doubled it. trillion-dollar deficits for the last four years. >> not true. the annual deficit was already running at $1.2 trillion when obama took office. >> reporter: how is fact checking actually done? on the last presidential debate, we watched one of the oldest fact-checking groups do their job. they are called factcheck.org, established by the university of pennsylvania. four members went to work. eugene kiely used to be a newspaper reporter. he covered congress and government for more than 20 years. he is managing this evening's fact checking. been as important or central in this campaign as the economy has been up until recently, there's some material here that we're going to have to research for first time. >> reporter: as soon as the debate begins, kiely starts to type what both candidates say. he comes across figures or comments that need checking. next, he e-mails colleagues, telling them what to research. then they check the accuracy of the comments. they use official sources like those of the congressional research for candidates'